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Acute Hamstring Injuries

Hamstring injuries are the most common non-contact injury in Australian Rules football (Opar, Williams & Shield 2012). The mechanisms behind hamstring injuries are numerous. They can include kicking, tackling, cutting, slow-speed stretching and many more; however the majority of hamstring injuries, especially in sport occur during running. The athlete will present with a sharp, local pain in the posterior thigh. This can range from mild to severe based on the severity of the strain and the amount of disruption occurring in the muscle belly. As with all soft-tissue injuries, the severity of the injury is classified as following:

Grade 1: Microscopic tearing of the muscle belly with minimal loss of function.

Grade 2: Moderate tearing of the muscle belly with moderate loss of function.

Grade 3: Complete rupture of the muscle belly with complete loss of function.

Hamstring injuries can result in significant time lost from training and competition. Furthermore, studies on Elite AFL players have demonstrated a significant decline in player performance following return from a hamstring injury.

Anatomy

The hamstring group of muscles consists of the Biceps Femoris, Semimembranosus and Semitendinosus. The Biceps Femoris is a two-headed muscle- the long head arises from the Ischial Tuberosities, or ‘sit-bones’ in addition to the Semimembranosus and Semitendinosus. The short-head of the Biceps Femoris muscle arises from the Linea Aspera, which lies along the posterior surface of the Femur. The Biceps Femoris muscle inserts laterally on the head of the Fibula. The Semitendinosus and Semimembranosus insert medially to the Pes Anserinus and Medial Tibial Condyle respectively.

The Biceps Femoris is the most commonly injured of the hamstring muscles, with the musculo-tendinous junction being the most common site of tissue damage (Opar, Williams & Shield 2012).

Hamstring Function in Running

Knowledge of the actions of the hamstrings muscle is essential for successful and functional rehabilitation following a hamstring strain. I will outline the key points of hamstring muscle function with reference to this diagram.

The hamstrings are active throughout the entire walking and running gait cycle. Muscle activation peaks at the early stance (between initial contact and mid-stance) and terminal swing phases.

Early Stance Phase:

During early stance phase, the hamstrings contract forcefully to extend the hip. This allows the build-up of rapid momentum which occurs during sprinting and running.

Terminal Swing Phase:

During terminal swing, the hamstrings are forced to contract eccentrically, functioning to decelerate the extending knee and the flexing hip.

During terminal swing the hamstring muscles reach their peak in muscle length. The long-head of Biceps Femoris undergoes the greatest amount of stretch (~110%), whilst Semimembranosus and Semitendinosus reach 107.5% and 108.2% respectively.

It is at this point, where the muscle is most forcefully contracted in its most lengthened position that the muscle is most susceptible to damage.

Pathology of Hamstring Strains

As previously mentioned- the biarticular nature of the hamstring muscles predisposes them to rapid changes in muscle length. Significantly, the hamstring muscles are at their most lengthened position during combined hip flexion and knee extension. This lengthening can exceed the mechanical limits of the hamstring muscle leading to strains and the accumulation of microscopic muscle damage- especially when lengthening is combined with forceful contraction.

The Biceps Femoris is the most commonly injured of the hamstrings group of muscles. The two heads of this muscle have different innervation. The long-head of the Biceps Femoris is innervated by the Tibial Portion of the Sciatic Nerve. The short-head of the Biceps Femoris is innervated by the Common Fibular Branch of the Sciatic Nerve. It is thought that this dual innervation can cause uncoordinated contraction of the two heads of this muscle leading to strain on the muscle.

The hamstring muscles predominantly consist of Type II muscle fibers (~58%). It is hypothesised that Type II muscle fibers are more susceptible to damage in an eccentric contraction.

Risk Factors:

Degree of Anterior Pelvic Tilt: The causes and implications of anterior pelvic tilt have been discussed here. The hamstrings share the common origin of the Ischial Tuberosity. In the instance of anterior pelvic tilt, the Ischial Tuberosity is elevated which in turn places the hamstrings at a greater resting length. When the already stretched hamstring muscle undergoes further lengthening in the terminal swing phase there is a greater chance of injury.

Previous Injury: A number of studies have documented that athletes with previous hamstring injuries are at risk of subsequent hamstring injuries. Firstly, this may indicate the presence of risk factors which have not yet been ameliorated- for instance, anterior pelvic tilt. Secondly, previous injury can lead to the formation of scar tissue. This is associated with reductions in eccentric strength, alterations in muscle lengthening mechanics and reduced flexibility.

Hamstrings-to-Quadriceps Muscle Strength Ratio: During running and kicking, the quadriceps rapidly contract to accelerate the lower leg into extension. The hamstrings then act as a ‘brake’. It is thought that relative eccentric hamstring weakness in relation to concentric quadriceps can place an athlete at risk of a hamstring injury.

Preventing Hamstring Injury

Eccentric Hamstring Exercise: Elite Soccer teams who implemented Nordic Hamstring Exercises (NHE) displayed significantly lower rates of hamstring injury. The athletes were asked to perform the following exercise 3x per week during the off-season before training, and 2x per week following training during the regular season:

The players were instructed as follows: “This is a partner exercise, in which your partner stabilizes your legs. The goal is to hold as long as possible, to achieve maximum loading of the hamstrings during the eccentric stage. Lean forward in a smooth movement, keep your back and hips extended, and work at resisting the forward fall with your hamstring muscles as long as possible until you land on your hands. Touch down with your hands, go all the way down so that your chest touches the ground and forcefully push off into a kneeling position with minimal concentric load on the hamstring. Load is increased by attempting to withstand the forward fall longer. When you can withstand the whole range of motion for 12 repetitions, increase load by adding speed to the starting phase of the motion. Initial speed and load can also be increased further by having someone push you at the back of your shoulders.”

Flexibility Training: To date, no studies have indicated flexibility training as a measure to reduce the incidence of hamstring injuries. There is poor evidence which suggests that poor flexibility is a risk factor for hamstring injuries.